1. Field of the Invention
This invention relates to integrated circuit structures. More particularly, this invention relates to an integrated circuit structure having an intermediate metal silicide layer.
2. Background Art
In the production of integrated circuit structures, such as, for example, static RAM chips, metal step coverage of contacts in the application of the final layers or rear end of the process is less than satisfactory when an intermediate metal layer is used. The presence of such an intermediate metal layer complicates the step coverage problem because higher temperature processing cannot be used to smooth out the step coverage since, if sufficient heat is used to soften the outer oxide layer to smooth out the step, the heat may adversely affect the intermediate metal layer.
Furthermore, it is customary to provide an undoped oxide layer over a polycrystalline silicon (hereinafter referred to as polysilicon or poly) layer having lightly doped portions to provide resistive paths or poly loads. This second, undoped, oxide layer is applied to prevent diffusion of the dopants from getting to the poly load area of the polysilicon layer. The use of an additional doped oxide layer over this oxide layer to provide a lower melting point oxide whereby the step coverage can be smoothed out means that contact openings must penetrate three layers of oxide before contacting the desired portion of the silicon substrate, such as, for example, the drain or source of an MOS transistor. This, in turn, can provide inadequate metal coverage in the contact opening resulting in thinning of portions of the metal on the wall of the contact opening.
While the use of an intermediate conductive layer of metal silicide is not unknown, prior uses of such a metal silicide have been made by concurrently sputtering silicon and a refractory metal to form a metal silicide layer which, after selective etching, is covered by a polysilicon layer. Subsequent doping of the poly layer results in the formation of resistive paths between the metal silicide portions. However, this use of two conductive layers provides a further step which must be covered during the subsequent application of further layers.